CASUALTIES OF VEGETABLES. CHAP. Xlf. 



of the petiole become gradually more woody, as 

 well as the whole of the leaf. The sap by conse- 

 quence stagnates, and at last the bond of union 

 between the leaf and stem is dried up, and cracks. 

 The wound that the stem thus receives cicatrizes 

 before the petiole separates ; and the petiole sepa- 

 rates at last in consequence of the interrupted con- 

 nexion between the leaf and stem which the crack 

 has occasioned.* 



This, it must be confessed, does not make up for 

 the deficiencies of the hypothesis of Du Hamel; 

 for in the first place there is no proof that the bond 

 of union between the leaf and stem cracks in the 

 manner here supposed. And even upon the sup- 

 position of its being the fact, it is, in the second 

 place, extremely improbable that the petiole should 

 after the cracking of this bond of union still con- 

 tinue attached to the stem, till the wound thus oc- 

 casioned has cicatrized ; because when the original 

 bond of union cracks there remains no other bond 

 of union by which the petiole is to retain its hold. 



Another explication is that of Vorlick, as quoted 

 by Willdenow ; the leaf which possesses a peculiar 

 vitality within itself, though dependant upon the 

 vitality of the plant, and generally of shorter du- 

 ration, dies when it reaches maturity ; and the 

 plant, being able to exist for a time without leaves, 

 throws off the dead leaf as the animal throws off 

 the dead part from the sound part. 

 * Princip. of Bot. p. 305. 



